The invention relates to directional boring and, in particular to an improved bit for impact-assisted directional boring.
Directional boring apparatus for making holes through soil are well known. The directional borer generally includes a series of drill rods joined end to end to form a drill string. The drill string is pushed or pulled through the soil by means of a powerful hydraulic device such as a hydraulic cylinder, See Malzahn, U.S. Pat. Nos. 4,945,999 and 5,070,848, and Cherrington, U.S. Pat. No. 4,679,775 (RE 33,793). The drill string may be pushed and rotated at the same time as described in Dunn, U.S. Pat. No. 4,953,633 and Deken, et al., U.S. Pat. No. 5,242,026. A spade, bit or head configured for boring is disposed at the end of the drill string and may include an ejection nozzle for water to assist in boring.
In one variation, a series of drill string rods are used in combination with a percussion tool mounted at the end. The rods can supply a steady pushing force to the impact and the interior of the rods can be used to supply the pneumatic borer with compressed air. See McDonald et al. U.S. Pat. No. 4,694,913. This system has, however, found limited application commercially, perhaps because the drill string tends to buckle when used for pushing if the bore hole is substantially wider than the diameter of the drill string.
Accurate directional boring necessarily requires information regarding the orientation and depth of a cutting or boring tool, which almost inevitably requires that a sensor and transmitting device (xe2x80x9csondexe2x80x9d) be attached to the cutting tool to prevent mis-boring and re-boring. One such device is described in U.S. Pat. No. 5,633,589, the disclosure of which is incorporated herein for all purposes. Baker U.S. Pat. No. 4,867,255 illustrates a steerable directional boring tool utilizing a pneumatic impactor.
Directional boring tools with rock drilling capability are described in Runquist U.S. Pat. No. 5,778,991, Cox U.S. Pat. No. 5,899,283 and Wentworth et al. PCT Publication No. 00/55467, published Sep. 21, 2000. The system of the foregoing PCT publication describes a bit for use in a drill head provided with a pneumatic impactor and a sonde housing. A gage tower sticks out radially from an otherwise generally round bit body defining the main cutting surface, giving the bit a keyhole shape. Loose stones from the wall of the borehole tend to fall into the spaces between the gage tower and the main cutting surface of the bit, especially during steering, sometimes causing the bit to jam.
FIG. 1A-1C illustrate a keyhole-shaped bit 5 in a series of positions defining a cutting arc for steering through a rocky formation according to the foregoing PCT publication. A stone 6 is in a jamming position next to the gage tower 7. The distance M1 represents the maximum clearance between the bit and the wall of the borehole 8 being drilled. This clearance has been measured as 0.46 inch for a bit sized to drill a 4 inch hole, and represents approximately 23% of the 2 inch radius of the gage cutting teeth 9 from the bit""s axis of rotation. The present invention reduces this clearance without otherwise compromising bit performance.
A drill bit for directional boring according to the invention includes a bit body having a frontwardly facing, circular main cutting surface having a number of main cutting teeth disposed thereon, a gage cutter extending radially outwardly from the main cutting surface, which gage cutter mounts at least one frontwardly facing gage cutting tooth thereon suitable for cutting in a circle during straight boring and over an angle defined by less than a full rotation of the bit in order to steer the bit, and a heel on an outer side surface of the bit body at a position opposite to the gage tooth, which heel provides a reaction surface for the gage cutter. The bit body and gage cutter have a nearly circular side profile along a front portion thereof, such that a maximum radial clearance between the side profile and a side wall of a hole being drilled is not greater than about 10 percent, preferably from 2% to 7%, of the radius of the circle cut by the gage tooth.
The side profile may be defined by the gage cutter alone, if the gage cutter extends all the way around the main cutting surface, or by the gage cutter and bit body in combination, if the gage cutter extends part way around the main cutting surface. The side profile is preferably not perfectly circular in cross section, and may instead be slightly ovoid or a composite of two or more arcs (radii) that merge together to resemble an oval or ellipse. The side profile is rounded around the entire periphery of the bit (360xc2x0), except at locations where small grooves or flats are provided as channels. These small indentations in the side of the bit body have limited width and depth and do not tend to cause jamming problems, and therefore are not considered when computing the maximum radial clearance of the side profile. Providing such a rounded profile on the side of the bit near its front end reduces or eliminates problems with jamming due to loose stones or rocky debris.
As in the prior Wentworth et al. PCT publication, the main cutting surface of the bit is preferably substantially flat and circular and has fluid ejection ports thereon, and the bit body has passages for conducting a drill fluid there through to the ejection ports. The heel of the bit may be provided by the metal of the bit body, protruding studs, or a combination thereof, and serves to provide a clearance by which the bit can rise in the steering direction as described further hereafter. In a preferred embodiment, the undersurface of the bit body at the heel is slightly inclined inwardly (towards the bit""s axis of rotation) to make it easier for the bit to rise in as it travels forward.
For purposes of the invention, the terms xe2x80x9cgenerally roundxe2x80x9d or xe2x80x9cgenerally circularxe2x80x9d include a cross-sectional shape that is truly round, or one that looks approximately round but is actually ovoid, elliptical or a composite of arcs that blend with one another each based on different diameter circles. A bit with a gage tower having side walls that stick out from an otherwise circular rim, as shown in FIG. 35 of the foregoing Wentworth et al. PCT application and FIGS. 1A-1C described above, is not generally circular by this definition. The meaning of xe2x80x9cnearly roundxe2x80x9d or xe2x80x9cnearly circularxe2x80x9d according to the invention is the same as xe2x80x9cgenerally roundxe2x80x9d or xe2x80x9cgenerally circularxe2x80x9d, except that a completely round shape is excluded. These and other aspects of the invention are discussed in the detailed description that follows.